Add derive for Schema

[Ten0]

Fixes #3

Introduces two new crates:

• serde_avro_derive the user-facing crate that contains the necessary traits/structs and reexports the macro
• serde_avro_derive_macros the proc-macro crate that contains the actual derive (not user-facing)

The new traits and construction structs are not in the main crate for several reasons:

• serde_avro_fast can be built before serde_avro_derive, which improves compilation time
• The API of serde_avro_derive is not deemed stable and/or private enough for the 1.0 release of serde_avro_fast